1. Field of the Invention
The present invention relates to an image acquisition apparatus and a resolution enhancing method which generate high-resolution images at a high frame rate using an image sensor which can read at different resolutions. Further, the invention relates to a computer readable recording medium which stores a program including a command for allowing a computer to execute the resolution enhancing method.
2. Description of the Related Art
Due to the development of semiconductor techniques in recent years, the number of pixels of image sensors tends to be increased. Although the devices are miniaturized and sped up, a lot of time is required for reading data from an image sensor whose number of pixels is increased. When the number of pixels is increased in one area, since the size of one pixel with respect to the size of the image sensor becomes small, for example, an S/N ratio is deteriorated. For this reason, in an image sensor with a larger number of pixels, it is difficult to shoot moving images at a high frame rate.
On the other hand, apparatuses for taking both still images and moving images are commercialized in recent years. In such an apparatus, a high-resolution image sensor is used, and still images are shot with a high resolution. On the other hand, in shooting moving images, a data reading mode called a draft mode is used, since moving images should be shot at an increased frame rate. For example, a reading method where thinning-out reading for reading four of eight vertical pixels for two clocks in CCD and skipping the rest is combined with plural field reading, or a method for adding pixels in horizontal and vertical directions, as in an MOS image sensor is used. In such a manner, one image sensor is shared in shooting both still images and moving images. Examples of actual products are image acquisition apparatuses such as digital cameras and digital video cameras which can shoot still images during shooting of moving images. These image acquisition apparatuses can shoot still images using an interruption signal by pressing a shutter button during the shooting of moving images.
In these image acquisition apparatuses, low-resolution images read for moving images are obtained as an image string with a high frame rate. However, since a pixel mixing technique or the like as described above is used, the resolving power of the image sensor cannot be exerted. For this reason, even when the moving images obtained by the shooting of the moving images include an image which is desired to be recorded as a high-resolution still image or an image which is desired to be printed finely, namely, a so-called right-moment scene image, such scene image with a high resolution cannot be obtained. Needless to say, when the shutter button is pressed during the shooting of moving images, a high-resolution still image can be acquired at that moment. However, in shooting before and after the moment of the shutter pressing, as in high-speed continuous shooting and pre-capturing, low-resolution images at the time of shooting moving images are shot. In another technique, means for enhancing resolution by applying an interpolating process or the like to the low-resolution images is used, but high-resolution images of high quality cannot be acquired by the interpolating process or the like.
Therefore, it is desired that any image during the shooting of moving images is read as a still image whose resolution is enhanced to high quality.
Various methods for generating high-resolution images using image data with less pixels are proposed in using image acquisition apparatuses such as video cameras. In order to solve the above problems of (1) high-resolution high-speed shooting and (2) enhancement of moving image resolution, as one example, Jpn. Pat. Appln. KOKAI Publication No. 10-69537 discloses a method for generating a high-resolution image using a plurality of low-resolution images with displacement. In this resolution enhancing process, a plurality of images with displacement at a sub-pixel level are shot with uniform resolution, deterioration factors of the images are eliminated and the images are synthesized so that one high-definition image is formed.
Jpn. Pat. Appln. KOKAI Publication No. 2004-40422 discloses a system for shooting a plurality of low-resolution images and high-resolution images whose sampling rate is lower than the low-resolution images, so as to generate a high-resolution image from the two kinds of image data, using a data compressing technique adopting two kinds of resolutions. In this system, when a plurality of low-resolution images and high-resolution images are shot and a high-resolution image is generated using these images, displacement correspondences among the low-resolution images are obtained. These are obtained in the following manner. The displacements between adjacent frames in a plurality of low-resolution images are obtained as local corresponding Information. Thereafter, the plurality of pieces of local corresponding information are integrated at an interval of a key frame, so that global corresponding information is generated as displacement between the key frames selected from the low-resolution images.